Creating HTML content with a fixed aspect ratio without the padding trick

It seems to be a common problem, you want to display some content on the web with a certain aspect ratio but you don’t know the size you will be displaying at. How do you do this? CSS doesn’t really have the tools to do the job well currently (there are proposals). In my case I want to display a video and associated controls as large as possible inside a space that I don’t know the size of. The size of the video also varies depending on the one being displayed.

Padding height

The answer to this according to almost all the searching I’ve done is the padding-top/bottom trick. For reasons that I don’t understand when using relative lengths (percentages) with the CSS padding-top and padding-bottom properties the values are calculated based on the width of the element. So padding-top: 100% gives you padding equal to the width of the element. Weird. So you can fairly easily create a box with a height calculated from its width and from there display content at whatever aspect ratio you choose. But there’s an inherent problem here, you need to know the width of the box in the first place, or at least be able to constrain it based on something. In my case the aspect ratio of the video and the container are both unknown. In some cases I need to constrain the width and calculate the height, but in others I need to constrain the height and calculate the width which is where this trick fails.

object-fit

There is one straightforward solution. The CSS object-fit property allows you to scale up content to the largest size possible for the space allocated. This is perfect for my needs, except that it only works for replaced content like videos and images. In my case I also need to overlay some controls on top and I won’t know where to position them unless they are inside a box the size of the video.

The solution?

So what I need is something where I can create a box with set sizes and then scale both width and height to the largest that fit entirely in the container. What do we have on the web that can do that … oh yes, SVG. In SVG you can define the viewport for your content and any shapes you like inside with SVG coordinates and then scale the entire SVG viewport using CSS properties. I want HTML content to scale here and luckily SVG provides the foreignObject element which lets you define a rectangle in SVG coordinates that contains non-SVG content, such as HTML! So here is what I came up with:

<!DOCTYPE html>

<html>
<head>
<style type="text/css">
html,
body,
svg,
div {
  height: 100%;
  width: 100%;
  margin: 0;
  padding: 0;
}

div {
  background: red;
}
</style>
</head>
<body>
  <svg viewBox="0 0 4 3">
    <foreignObject x="0" y="0" width="100%" height="100%">
      <div></div>
    </foreignObject>
  </svg>
</body>
</html>

This is pretty straightforward. It creates an SVG document with a viewport with a 4:3 aspect ratio, a foreignObject container that fills the viewport and then a div that fills that. what you end up with is a div with a 4:3 aspect ratio. While this shows it working against the full page it seems to work anywhere with constraints on either height, width or both such as in a flex or grid layout. Obviously changing the viewBox allows you to get any aspect ratio you like, just setting it to the size of the video gives me exactly what I want.

You can see it working over on codepen.

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